The tread of conventional vehicle tires typically comprises a plurality of load bearing tread block elements separated by a network of grooves. The tread block elements are arranged in a pattern around the circumference of the tire. As the tire rolls over a road surface the tread block elements move into engagement with the road surface at the leading edge of the tread's footprint and out of engagement at the trailing edge of the tread's footprint. This causes disturbances in the surrounding air molecules that generate sounds. For any given tread pattern a certain amount of acoustic energy is produced. The distribution of the acoustic energy over the audio frequency spectrum has been found to be controlled at least in part by the geometry of the tread block elements. Where tread block elements of constant pitch length have been used to completely surround a tire, the sound generated is dominated by a single frequency and its harmonics, i.e., a majority of the sound generated is concentrated into a small frequency range of the audio frequency spectrum. This tonal concentration may be highly disturbing to the occupants of the vehicle on which the tire is mounted.
Much effort has been directed to the reduction of objectionable tire noise, with emphasis being placed on the spreading of the energy produced by the tire tread pattern over the audio frequency spectrum thereby to reduce objectionable tonal concentration. A technique known as "pitching" accomplishes this by varying the length, i.e., the pitch, of the design cycle of the tread of the tire around its circumference, with the result being a "variable pitch" tire tread pattern. The design cycle is a representative portion of the tread which typically comprises in the circumferential direction at least one tread block element and a generally transversely extending groove. For conditions of practical manufacture of the molds for the tires the circumference of the tire is composed of a whole number of design cycles. The lengths of the design cycles may be varied in a random or preselected algorithmic manner to distribute the audio noise throughout the frequency spectrum.
Another known noise treatment technique is phase optimization at the leading and trailing edges of the tire's footprint. This technique involves circumferential rotation of one portion of the tread pattern relative to another portion, such as, for example, one circumferential rib relative to another circumferential rib. In this manner the tread block elements extending transversely across the tread may be caused to engage and disengage the road surface in staggered relationship. Computer simulations heretofore have been used to perform phase optimization, as well as optimal pitch sequencing.
While it is advantageous to distribute the noise energy over a wide range of frequencies, a more fundamental objective is the reduction of the total amount of sound energy that needs to be modulated. If the total amount of noise is reduced, the noise at each frequency will be correspondingly reduced with the result being a quieter tire. This is accomplished in European Patent Publication No. 0 357 462 by optimizing the inclined angle of transverse grooves dividing circumferential ribs into a plurality of block elements. It was discovered that sound energy rapidly dropped when the inclined angle took certain values. The applicant hereof observed that in at least one tread design these node points corresponded to a balancing of the void across the leading edge of the tire's footprint. Applicant also observed that in a tire having only circumferential grooves and straight sided circumferential ribs (i.e., a tire theoretically having no tread element impact noise) that the void across the trailing and leading edges was constant about the circumference of the tire.